Disposable pressware containers made from paperboard and so forth, such as plates, trays, bowls and the like are well known in the art. Typically, such articles are manufactured on an inclined die set having upper and lower halves. Illustrative in this regard is U.S. Pat. No. 5,249,946 to Marx assigned to the assignee of the present invention. Referring to the '946 patent, a typical product is manufactured by way of feeding a continuous paperboard web into a cyclically operating blanking section. The forming section includes a plurality of reciprocating upper die halves opposing, in facing relationship, a plurality of lower die halves. The upper die halves are mounted for reciprocating movement in a direction that is oblique or inclined with respect to the vertical plane. The blanks, after cutting, are gravity fed to the inclined lower die halves in the forming section.
Particular forming dies and processes for making pressed paperboard products are likewise well known. Most typically, dies sets for forming paperboard containers, for example, include a male or punch die half and a female die half. Typically, the punch half is reciprocally mounted with respect to its opposing die half and both die halves are segmented. One or more portions of the die halves may be spring-biased if so desired, and the particular geometry of the die will depend upon the product desired. In this regard, there is shown in U.S. Pat. No. 4,832,676 to Johns et al. an apparatus for forming a compartmented paperboard plate. The dies illustrated in the '676 patent includes spring-biased segments as well as pressure rings on the punch half and draw rings of the plate. The particular apparatus further includes articulated, full area knock-outs.
Forming operations can be somewhat critical in order to produce quality product at the desired rates. In this respect, U.S. Pat. No. 4,721,500 to Van Handel et al. is informative. Note also U.S. Pat. No. 4,609,140 to Van Handel et al. The '140 patent provides a general description of one forming method as will be appreciated from FIG. 3 thereof. FIG. 3 shows a cross section of the upper die half and lower die half which are utilized to press a flat, circular paperboard blank into the shape of the plate. The construction of the die halves and the equipment on which they are mounted is substantially conventional; for example, as utilized on presses manufactured by the Peerless Manufacturing Company. To facilitate the holding and shaping of the blank, the die halves are segmented in the manner shown. The lower die has a circular base portion and a central circular platform which is mounted to be moveable with respect to the base. The platform is cam operated in a conventional manner and urged toward a normal position such that it's flat top forming surface is initially above the forming surface of the base. The platform is mounted for sliding movement to the base, with the entire base itself being mounted in a conventional manner on springs. Because the blank is very tightly pressed at the peripheral rim area, moisture in the paperboard which is driven therefrom during pressing and the heated dies cannot readily escape. To allow the release of this moisture, at least one circular groove is provided in the surface of the base which vents to the atmosphere through a passageway. Similarly, the top die half is segmented into an outer ring portion, a base portion and a central platform having a flat forming surface. The base portion has curved, symmetrical forming surfaces and the outer ring has curved forming surfaces. The central platform and the outer ring are slidingly mounted to the base and biased by springs to their normal position shown in FIG. 3 in a commercially conventional manner. The top die half is mounted to reciprocate toward and away from the lower die half. In the pressing operation, the blank is first laid upon the flat forming surface, generally underling the bottom wall portion of the plate to be formed, and the forming surface makes first contact with the top of the blank to hold the blank in place as the forming operation begins. Further downward movement of the top die half brings the spring-biased forming surfaces of the outer ring into contact with the edges of the blank to begin to shape the edges of the blank over the underlying surfaces in the areas which will define the overturned rim of the finished plate. However, because the ring is spring-biased the paperboard material in the rim area is not substantially compressed or distorted by the initial shaping since the force applied by the forming surfaces is generally light and limited to the spring force applied to the ring. Eventually, the top die half moves sufficiently far down so that the platform segments and the ring are fully compressed such that the adjacent portions of the forming surfaces are coplanar. In a conventional manner the die halves are heated with electrical resistance heaters and the temperature of the die halves is controlled to a selected level by monitoring the temperature of the dies with thermistors mounted in the dies as close as possible to the forming surfaces.
For paperboard plate stock of conventional thicknesses in the range of from about 0.010 to about 0.040 inches, it is preferred that the spacing between the upper die surface and the lower die surface decline continuously from the nominal paperboard thickness at the center to a lower value at the rim.
The springs upon which the lower die half is mounted are typically constructed such that the full stroke of the upper die results in a force applied between the dies of from about 6000 to 8000 pounds.
The paperboard which is formed into the blanks is conventionally produced by a wet laid paper making process and is typically available in the form of a continuous web on a roll. The paperboard stock is preferred to have a basis weight in the range of from about 100 pounds to about 400 pounds per 3000 square foot ream and a thickness or caliper in the range of from about 0.010 to about 0.040 inches as noted above. Lower basis weights in caliper paperboard is preferred for ease of forming and to save on feedstock costs. Paperboard stock utilized for forming paper plates is typically formed from bleached pulp furnish, and is usually double clay coated on one side. Such paperboard stock commonly has a moisture (water content) varying from about 4.0 to about 8.0 percent by weight.
The effect of the compressive forces at the rim is greatest when the proper moisture conditions are maintained within the paperboard: at least 8% and less than 12% water by weight, and preferably 9.5 to 10.5%. Paperboard having moisture in this range has sufficient moisture to deform under pressure, but not such excessive moisture that water vapor interferes with the forming operation or that the paperboard is too weak to withstand the high compressive forces applied. To achieve the desired moisture levels within the paperboard stock as it comes off the roll, the paperboard is treated by spraying or rolling on a moistening solution, primarily water, although other components such as lubricants may be added. The moisture content may be monitored with a hand held capacitive type moisture meter to verify that the desired moisture conditions are being maintained. It is preferred that the plate stock not be formed for at least six hours after moistening to allow the moisture within the paperboard to reach equilibrium.
Because of the intended end use of the paper plates, the paperboard stock is typically coated on one side with a liquid proof layer or layers. In addition, for esthetic reasons, the paper plate stock is often initially printed before being coated. As an example of typical coating material, a first layer of polyvinyl acetate emulsion may be applied over the printed paperboard with a second layer of nitrocellulose lacquer applied over the first layer. The plate stock is moistened on the uncoated side after all of the printing and coating steps have been completed. In a typical forming operation, the web of paperboard stock is fed continuously from a roll through a cutting die to form the circular blanks which are then fed into position between the upper and lower die halves. The dies halves are heated as described above, to aid in the forming process. It has been found that best results are obtained if the upper die half and lower die half—particularly the surfaces thereof—are maintained at a temperature in the range of from about 250° F. to about 320° F., and most preferably at about 300° F.±110° F. These die temperatures have been found to facilitate the plastic deformation of paperboard in the rim areas if the paperboard has the preferred moisture levels. At these preferred die temperatures, the amount of heat applied to the blank is apparently sufficient to liberate the moisture within the blank under the rim and thereby facilitate the deformation of the fibers without overheating the blank and causing blisters from liberation of steam or scorching the blank material. It is apparent that the amount of heat applied to the paperboard will vary with the amount of time that the dies dwell in a position pressing the paperboard together. The preferred die temperatures are based on the usual dwell times encountered for normal production speeds of 40 to 60 pressings a minute, and commensurately higher or lower temperatures in the dies would generally be required for higher or lower production speeds, respectively.
As will be appreciated by one of skill in the art, the knock-outs are important for holding the container blank on center during formation and for separating the finished product from the die halves, particularly during high speed operation. The mechanical features can be further augmented pneumatically as is disclosed in U.S. Pat. No. 4,755,128 to Alexander et al.
Another important feature in paperboard press manufacturing of plates, trays, bowls and the like and of particular interest to the present invention is the paper positioning of the paperboard blank in the apparatus. There is disclosed in U.S. Pat. No. 4,435,143 to Dempsey a pressing apparatus for paperboard trays. The apparatus is equipped with blank stops in the form of upstanding members (185, see FIG. 3) that are configured to stop a blank, but to allow a formed container to slide therethrough.
So also U.S. Pat. No. 5,041,071 to Reasinger et al. discloses a paper tray forming machine with a blank centering device which consists of at least 1 finger which is moved upward and toward two passive stops (30A, 30B, FIG. 2). The finger may be shaped similarly to the blank edge so that the precise positioning occurs during motion of the fingers.
Likewise, U.S. Pat. No. 4,778,439 to Alexander discloses an apparatus for making paper clam shells including stationary stops 66 (FIG. 1, 4) and retractable stop pins 60.
Other methods that have been employed (although not believed to be prior art to the present invention) include blank stops of an angular construction in a left and right handed design as discussed further hereinafter. These type of stops can be used for circular, oval, rectangular style plates and trays. The fixed angular blank stops can be shimmed to a range of distances from the center in an attempt to compensate for blank diameter variations, bounce, angled blank transfer chute delivery or blank curl. Blanks gravity fed to the die set will contact the fixed angular stops with significant speed and energy. Blanks will often “bounce back” at least once before settling against the two stops. Sometimes the blanks do not return against one or both of the stops before the cycling die sets form the pressware product. Thus the product is formed off center, having a longer downturned area on one side of the product than the other. This differing downturned area will often reduce product strength. The off center formed products are rejected from die set and are stacked at the end of the conveyor. The stack product can have a non-uniformed appearing edges resulting from random product formation resulting from the blank bounce. Press forming speeds may be limited, especially for larger higher weight products which have more kinetic energy, blank bounce and require more time for final settling.
One possible blank stop technique involves the use of fixed non-rotating pin stops. These stops are easier and less expensive to machine than the fixed angle blank stops. They are tightly bolted to the die half or draw ring and the pin diameters are chosen to center the blank in the forming set. Differing pin diameters can be produced and installed in attempts to compensate for varying blank diameter, blank bounce, angled blank transfer chute delivery or blank curl. A set of two pins per sides (four pins total) may be used towards the front of the die set to stop the product. Four pins can be used to minimize blank damage due to the sudden impact of the blanks against the stops. Blank indenting can result with a non-rotating two or four pin system. Since the pins cannot rotate, the kinetic energy of the gravity fed blanks cannot be absorbed resulting in blank bounce, off center forming a non-uniformed stack product. The fixed non-rotating pin stop system is a lower cost alternative to the fixed angle stop system; however, press forming speeds may be limited with this stop system as well.
Before turning to a more detailed discussion of the present invention, a system utilizing two stationary forward rotating pin stops and two rearward movable rotating pin stops has been used for several years by the assignee of the present invention for production of oval platters. The two rearward rotating pin stops are stroked backward by a pneumatic cylinder for the purposes of positioning an oval blank. This system differs from the system described herein in that the present invention involves two sets of two (four total rotating pin stops) which are located towards the front of the die set. This system is primarily designed for circular blanks and pressware products but may have advantages for other shapes such as ovals, squares, rectangles, and so forth. Oval, square or rectangular products may also require side guides on the die sets. The rotating pin stop system in accordance with the present invention provides utility for matched metal forming of paperboard, plastic, paper plastic composites, etc., for disposable food serving containers.